Unit 2 Bonding Overview Covalent Bonding Ionic and

Unit 2: Bonding

Overview • • Covalent Bonding Ionic and Metallic Bonding Electronegativity Molecular Shape Polarity Ionic Crystals Network Solids Intermolecular Forces

Covalent Bonding • Bonds between atoms are formed through the sharing of electrons • Covalent bonds form between two non-metal atoms through sharing of pairs of electrons • Atoms have a “desire” to have their outer energy levels filled (Octet Rule) • Covalent bonding can be represented with Lewis Dot Diagrams

Lewis Dot Diagrams • Lewis Dot Diagrams show the sharing of electrons between atoms and where the bonds form • atoms share electrons to fill their outer energy levels (8 electrons in their outer shell) • The exception is hydrogen (2 electrons in its outer shell)

Lewis Dot Diagrams for Hydrogen and Chlorine Gas The first row shows the atoms before they are bonded The second row shows the sharing of electrons to fill the outer energy level The third row has circles around the electrons to show those that belong to each atom. Where the circles overlap represents a covalent bond

Multiple Bonds • Double and triple bonds can form between atoms in order to fill the outer energy level • This occurs when two atoms share more than one pair of electrons

Multiple Lewis Structures • Some molecules can have more than one possible Lewis structure, usually when one single bond and one double bond can be exchanged within the rules of drawing Lewis structures • Example of SO 2 (g)

Structural Diagrams • Lewis Diagrams can be converted to structural diagrams for convenience • Structural diagrams use lines to represent a bond, or a pair of electrons, but it does not show lone electron pairs • Example: Chlorine Gas

Lewis Dot Diagram Worksheet • Using the rules for drawing Lewis dot diagrams, complete the worksheet (LDD and structural) • For extra practice, try the Lewis Structures Thought Lab

Stereochemistry – The Structures of Molecular Compounds • So far we have seen molecules represented in 2 -D • However, molecules are actually 3 dimensional • To predict 3 dimensional molecular shapes we use VSEPR theory (Valence-Shell Electron-Pair Repulsion) • Based on the electrostatic repulsion of electron pairs

• Note that the repulsion force is strongest between two lone pairs and the weakest between two bonded pairs, and the repulsion between a lone pair and a bonded pair is intermediate • We apply the VSEPR theory to a central atom that has an octet of electrons in its valence shell, and there are three categories of shapes; linear, trigonal planar, and tetrahedral • In VSEPR, an electron group is a bond (single or multiple) or a lone pair

Linear • Forms when a central atom has two electron groups • The shape is linear because the electron groups try to arrange themselves as far apart as possible • The bond angle between the electron groups is 1800 • The central atom is bonded to two other atoms by two double bonds or a combination of a single bond a triple bond

Trigonal Planar • A central atom with three electron groups has a trigonal planar shape • The bonding angle between the electron groups is 120 o • The central atom is either bonded to three atoms(trigonal planar), or two atoms and a lone pair(bent or V-shaped)

Tetrahedral • A central atom with four electron groups has a tetrahedral shape • The bonding angles between the electron groups is 109. 5 o • The central atom can be bonded to four atoms (tetrahedral), three atoms and a lone pair (trigonal pyramidal), or two atoms and two lone pairs (bent or V-shaped)

Information Summary # of lone electron pairs around the central atom # of bonds around the central atom Shape of the molecule Bond angles around the central atom 0 4 Tetrahedral 109. 5 0 3 Trigonal Planar 120 0 2 Linear 180 1 3 Pyramidal 107. 3 2 2 V-shaped or bent 104. 5 1 2 V-shaped or bent 119. 5

Steps to Predicting Molecular Shapes 1. Draw a primary LDD of the molecule 2. Determine the total number of electron groups around the central atom 3. Determine the types of electron groups (bonding pairs or lone pairs) 4. Determine which shape will accommodate the combination of electron groups

Electronegativity • Electronegativity is a measure of the relative ability of an element’s atoms to attract the shared electrons in a chemical bond. • Higher electronegativities mean a greater attraction for the electrons. • Fluorine is the highest with a value of 4. 0

Electronegativity • For main group elements, electronegativity tends to increase with the group number (left to right on the periodic table). • Notice the noble gases do not have electronegativities, why is that? • Electronegativities also increase as you move vertically up a group number.

Atom Size • For any period in the periodic table, as you move right the size of the atom decreases • Why does this trend exist?

Atom size • As the number of protons increases, the force attracting the electrons increases • The electrons are pulled closer to the nucleus of the atom

Polarity • The difference of electronegativity between two bonding atoms can be measured by subtracting the smaller number from the larger number. • The difference in the two electronegativities determines the nature of the bond

Polarity • Bonds that are sharing electrons UNEQUALLY between two atoms are called POLAR COVALENT BONDS • If the atoms are identical (equal electronegativity), the bond will not be polar. This is called NON-POLAR COVALENT BONDS

• Polar covalent bonds have a positive “pole” and a negative “pole” so they are also referred to as “bond dipoles” • Polar covalent bonds have an electronegativity difference between 0 and 1. 7 • Ionic bonds have an electronegativity difference between 1. 7 and 3. 3

Polar Molecules • Are molecules that contain polar bonds necessarily polar? • Examples of H 2 O and CO 2 • To determine if a molecule is polar we need to look at the overall direction of polarity • Draw in polarity arrows on your molecule and determine if the molecules are polar or not

Ionic Bonds • Ionic bonds form from the electrostatic attraction between oppositely charged ions • Atoms become ionic by losing or gaining electrons from the atom it is bonding with • Remember that an atom will lose its electrons to fill its outer level if its valence level is less than half full, as it is with metals

Electron Exchange and Ionic Bond Formation

Metallic Bonding • Metals can form bonds with other metals, but it is neither covalent or ionic • Metals cannot share electrons to form an octet of electrons around each atom • Imagine 8 sodium atoms all trying to share the same 8 electrons

• Although metal atoms do not form covalent or ionic bonds with each other, there must be relatively strong attractive forces holding the atoms together or else the metals would be in a gaseous state • In metallic bonding, the valence electrons are delocalized, which means they are free to move from one atom to the next

• Because the electrons are free to move, all of the atoms share all of the valence electrons • It is the electrostatic force between the positively charged metal ions and the negative electrons that make the metallic bond

Ionic Crystals • Rather than one metal bonding to one nonmetal, ionic substances have their ions packed together in a crystal lattice • The crystals can also be represented in a ball and stick model

• The sticks represent the attractive forces between the ions • Since all the attractions are equal, there are no pairs of ions to be identified as molecules • Therefore the formula only represents the ratio of the ions in the crystal

• The smallest ratio of ions in the crystal is called a formula unit, not a molecule • Shape of the macroscopic crystals is determined by the way their ions pack together • The smallest set of ions in a crystal needed to make the pattern is called a unit cell

Table Salt Sucrose Uncut Diamond

• The size of each ion influences the pattern of ions • Another influence is the relative charge of the ions, and therefore the ratio of ions in the crystal

Crystal formation • Many beautiful crystal formations can be found in nature as well as in the laboratory

Network Solids • Like ionic crystals, but they are held together with covalent bonds • Single elements can form an array of different network solids • Eg. Carbon forms graphite, diamond, nanotubes (pg. 60)

• Each different network solid of the same element just has different arrangement of the atoms • Some network solids contain two different elements • Eg. Silicon Dioxide (sand, quartz)

Questions

Intermolecular Forces • Covalent bonds exist between atoms within a molecular compound • These covalent bonds are called intramolecular forces • Forces holding entire molecules together are called intermolecular forces

Dipole-Dipole Forces • Occurs between polar molecules • Polar molecules have a positive pole and a negative pole so they are called dipoles • When two dipoles come close to each other, the positive pole of one is attracted to the negative pole of the other

Dipole-Dipole Attractions

• Each molecule can be attracted to four or more other polar molecules at the same time • This is called Dipole-Dipole Attraction • Not as strong as ionic attraction, but can be strong enough to stabilize a solid crystal • Eg. Table sugar

Hydrogen Bonding • Special dipole-dipole attraction that occurs between hydrogen and highly electronegative atoms such as oxygen, nitrogen, or fluorine • The positive nucleus of the hydrogen atom is attracted to the slightly negative charge on the other atom • Much stronger than other dipole-dipole attractions

Hydrogen Bonding in Water • Hydrogen bonding is an important factor that influences the structure and properties of water • One oxygen atom can be hydrogen bonded to as many as 6 other hydrogen atoms in other water molecules

Hydrogen Bonds in Ice • Each water molecule is hydrogen bonded to four other water molecules • The water molecules in ice are farther apart than in liquid water, therefore ice is less dense than liquid water** • Hydrogen bonds are the strongest in the form shown in the next diagram

Unique Properties Reading • Read the handout on the unique properties of water

London Dispersion Forces • Dispersion forces act between all molecules, but in non-polar molecules they are the only force • Even though there are no permanent dipoles in non-polar molecules, it is possible to induce dipoles

• Non-polar molecules also spontaneously form temporary dipoles • Electrons are in constant, rapid motion • For a brief moment the electron distribution can be uneven • This can form a positive pole and a negative pole in the molecule

• The temporary dipole in the molecule can induce a temporary dipole in the next molecule, like the balloon and the wall • The process disperses through the substance

Factors Affecting Magnitude • Two factors affect LD forces: – Increased electrons = increased probability of a temporary dipole forming – Linear shapes have greater London dispersion forces

Structures and Properties of Compounds • The state of a substance (solid, liquid or gas) depends on the strength of the intermolecular forces • As particles gain kinetic energy (heat) they break their intermolecular bonds and change state

Time of Hydrogen Bonding • FYI – hydrogen bonds in liquid water break and reform 100 000 000 (1011) times every second

Melting and Boiling Points • Melting and boiling points of ionic substances and metals are about the same magnitude • Melting and boiling points of molecular substances are much lower • What does that tell us about the forces?

• Ionic bonds are much stronger if the ions have a large charge • So ionic compounds that have ions with large charges will have higher melting/boiling points

Molecular Forces • Of the molecular compounds, dipoles that form hydrogen bonds are the strongest • Dipole-dipole forces are weaker than hydrogen bonds • Non-polar molecules that have London dispersion forces are the weakest

• Therefore, hydrogen bonded substances have the highest melting/boiling points, dipole have lower melting/boiling points, and non-polar substances have the lowest melting/boiling points

Mechanical Properties of Solids • Metals are malleable because of the nature of metallic bonds (positive ions in a sea of electrons) • Ionic substances are brittle because if a layer of the crystal is shifted down one position, like ions will be aligned and repel • Non-polar molecular substances are usually soft and easily broken

Conductivity • The ability of a substance to transfer electrical current • For this to occur, charged particles (ions or electrons) must be able to move freely

• Metals are good conductors because their electrons are free to move throughout the metallic structure • Ionic solids do not conduct electric current because the ions are held together in a rigid structure

• Ionic substances that are dissolved in water are good conductors because the charged ions are free to move in the solution • Some network solids can conduct electricity because of delocalized electrons (graphite) • Molecular compounds cannot conduct electricity in pure form or dissolved in water

Review Questions • Pg. 137 – #3, 4, 6 -14, 16, 19, 21, 22, 25 -29, 31, 34 -39, 41, 43, 47, 48, 51, 57, 60 – brdebenham@cbe. ab. ca

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